As is known in the art, in order to maintain a high degree of vacuum in a sealed vacuum container, such as for example in so called a Dewar assembly, a getter has been used to trap gas molecules that slowly leak through the Dewar assembly seal or outgas from the container material. Widely used getter materials include titanium alone, and mixtures of titanium zirconium, vanadium, iron, and other reactive metals, which permanently capture various gas molecules such as oxygen, hydrogen, nitrogen, methane, carbon monoxide and carbon dioxide that are typically found in an outgassed vacuum-sealed Dewar assembly. The getter materials react with these gases to form oxides, carbides, hydrides and nitrides which are stable at room temperature. Therefore, the reactions are irreversible and do not involve the risk of future gas release.
There are two categories of vacuum getters, Evaporable getters and non-evaporable getters. Evaporable getters are flash evaporated in place onto the interior Dewar surface after the Dewar is sealed. A prime example is the shiny surface seen in a glass radio or TV vacuum tube. If subsequently exposed to air, the getter cannot be reactivated. A non-evaporable getter is installed or deposited in the process of fabricating the device in which it will serve, and activated by heating it to a high temperature for a short time. The subject of this application is in the non-evaporable category.
Trapping of residual gas molecules in a Dewar assembly has been achieved by conventional externally fired getters, an example of which is described in U.S. Pat. No. 5,111,049, inventors Romano et al. A getter material such as a porous mixture of titanium and molybdenum powders is placed within an Alloy 42 container, which is welded onto a tube protruding from the Dewar body. The getter material is activated by applying heat to the getter container at about 800 degrees C. for about 10 minutes. However, the externally fired getter is large and bulky, and must be fabricated external to the Dewar body. To maintain a high degree of vacuum in a Dewar assembly that contains a modern planar Infrared (IR) detector array, which is typically rectangular with dimensions generally on the order of 0.5 to 2 cm, the use of an externally fired getter greatly increases the volume and weight of the assembly. Moreover, the getter material must be located away from the IR detector array, and external cooling must be applied to the Dewar body to prevent thermal damage to the detector array and other Dewar assembly components caused by the heat supplied to the getter. The mechanical complexity of the getter assembly and the need for an external cooler for the IR detector array increases the cost of the IR detector.
A process for fabricating the vacuum-sealed Dewar assembly is described in U.S. Pat. No. 5,433,639. However, since the surface area of the deposited thin film getter is small, the amount of gas that can be removed by the getter is limited. Because the IR detectors preferably have a large fill factor which is the ratio of the detector surface area to the total detector surface area to increase the effectiveness of detection, the percentage of surface area upon which the getter material can be deposited is therefore relatively small.
As is also known in the art, a conventional uncooled IR detector array is housed in a vacuum-sealed Dewar assembly with a planar IR window, usually made of germanium and coated with a surface coating to improve its IR transmittance. IR radiation passes through the window and strikes the detector pixels in the array. Uncooled IR detectors are typically silicon or Vanadium Oxide microbolometers, which are temperature sensors that detect IR radiation by heat sensing.
As is also known in the art, integrating a getter into a wafer level vacuum packaged (WLVP) device that requires a large area optical window is very limited in available area to place the getter. In a wafer level packaged device the getter is usually vacuum deposited by evaporation or sputtering the getter material onto the device lid. In an optical device, such as an IR imaging Focal Plane Array (FPA), the window occupies most of the available area onto which the getter would be deposited.
One technique is described in U.S. Pat. No. 5,701,008. As described, therein, an increase in the surface area of getter is achieved by etching a multitude of trenches to form column-like protrusions in the cap wafer surface where the getter is to be placed. The getter is deposited conformally on the convoluted surface, thereby increasing its surface area by adding a third dimension to the two-dimensional surface area. The getter is deposited conformally by evaporation or sputtering onto the walls of the column-like protrusions as well as the planar horizontal surfaces. Other attempts involve methods to roughen the surface to increase the area slightly before depositing a getter.
As is also known in the art, one method for forming a getter is to sputter a film comprised of Zr, Ti, Fe and other metals co-deposited on a substrate.
As is also known in the art, a deposited vacuum getter is a structure (usually a thin layer) which is formed by evaporating or sputtering a layer of material that can react chemically with residual gas atoms in a vacuum environment to improve the vacuum quality. The morphology of the getter film is important as it must have as large an effective surface area as possible, onto which reactive gas species will be trapped. The gettering area is not only the geometrical area. Most of the active area is provided by the voids at the grain boundaries. The growth of deposited films has been studied extensively, resulting in the well-known Structure Zone Models (SZMs) of Movchan and Demchishin, and Thornton, see Handbook of Deposition Technologies for Thin Films and Coatings, P. M. Martin, Elsevier, 2009, ISBN 978-0-8155-2031-3. The SZM models relate film structure to the homologous temperature, defined as the ratio of film growth temperature to the melting temperature of the deposited material. A critical factor in the film grain growth is the mobility of the arriving atom on the substrate surface. The mobility has a strong dependence on the arriving energy and surface temperature. Atoms with high mobility (high energy) will move and agglomerate on the surface and form large grains. Atoms with low energy will stop sooner and form smaller grains, resulting in a net larger void space than in a film with large grains. Thus a film with many small grains is preferred over one with large grains with void spaces between them. A fast deposition rate also promotes smaller gains with void spaces in between grains. The chemical and thermodynamic properties of the material being deposited also will have an influence on the resultant gain structure.
This can be illustrated in FIGS. 1A, 1A′-1C, 1C′ for the high mobility case with low deposition rate, and FIGS. 2A, 2A′-2C, 2C′ for the lower mobility case with higher deposition rate. In FIGS. 1A, 1A′ atoms 4 arrive on a surface 3 and move around until they lose enough energy to stop, or hit the edge of a cluster 6 of atoms which are the basis for forming a grain. Resultant grains are large as clusters grow sideways until they cover enough of the surface 3 to intercept an increasing number of arriving atoms 4 and start growing upward. The contact boundaries between grains 6 contains the void space 1 responsible for gettering action. In FIGS. 2C-2C′, atoms 4 arrive on surface 3 and move around until they lose enough energy to stop, or hit the edge of a cluster. Clusters start growing upward quickly as atoms 4 arrive fast enough to pile up and quickly cover much of the surface and thus form small grains with grain boundaries (void space) 1 between them.
As is also known in the art, the effectiveness of vacuum deposited getters is strongly dependent upon the deposition method, deposition conditions, and resultant film morphology and structure. Vacuum getters for WLP and some other electronic packages consist of a layer of metal deposited in the package in a way that the gain structure forms tall columnar structures. The vertical surfaces between the grains are many times the geometrical area of the deposited getter and constitute most of the gettering surface.